A boost converter circuit converts a relatively low input voltage to a relatively high output voltage and supplies it to a load. FIG. 1 shows a conventional boost converter circuit 10, which converts a relatively low input voltage Vin to a relatively high output voltage and supplies it to an output terminal Vout. In the circuit shown in FIG. 1, if the output terminal of the boost converter circuit 10 is shorted to ground, although the switch SW can be turned OFF by the control circuit 12, the path from the input terminal Vin to the output terminal Vout is still conductive; therefore, the inductor L still keeps storing energy, and there may be other damages caused by current continuously flowing on the path. Hence as shown in FIG. 2, preferably, a short circuit protection circuit is provided between the output VBO of the boost converter circuit 10 and the output terminal Vout, for cutting off the conduction between the node VBO and the output terminal Vout when the output terminal of the boost converter circuit 10 is shorted to ground.
FIGS. 3-7 show several conventional short circuit protection circuits 14. Referring to FIG. 3, in normal operation, the output voltage VBO is higher than the input voltage Vin, the gate to source voltage Vgs of the PMOS transistor Q being negative, so the PMOS transistor Q is completely ON, and the voltage at the output terminal Vout is substantially equal to the output voltage VBO of the boost converter circuit 10. When the output terminal Vout is shorted to ground, the boost converter circuit 10 does not operate, and the output voltage VBO of the boost converter circuit 10 drops. When the output voltage VBO is equal to the input voltage Vin, the gate to source voltage Vgs of the PMOS transistor Q becomes zero, and therefore the PMOS transistor Q turns OFF, cutting off the conduction between the node VBO and the output terminal Vout, to thereby provide the short circuit protection effect.
The operation of the circuits shown in FIGS. 4-6 is similar to that of FIG. 3. As to the circuit of FIG. 7, in normal operation, the switch S is ON; when the output terminal Vout is shorted to ground, a control signal is generated (for example from the comparator shown in FIGS. 1 and 2) to turn OFF the switch S, so that the gate to source voltage Vgs of the PMOS transistor Q becomes zero. In this manner, it also achieves the short circuit protection effect as above.
The conventional short circuit protection circuits shown in FIGS. 3-7 do provide the short circuit protection effect as required, but they have the following disadvantages. Because the circuit 10 is a boost converter, the output voltage VBO is a high voltage, and thus at the initial instant period when the output terminal Vout is shorted to ground, the source to drain voltage difference of the transistor Q is very large. For this reason, the transistor Q has to be made of a costly high voltage device capable of sustaining relatively high voltage, and it also increases the complexity of the corresponding wafer manufacturing process.
Therefore, a short circuit protection circuit made of relatively low voltage devices but capable of sustaining relatively high voltage is desired.